1. Field of the Invention (Technical Field)
The present invention relates to measuring transfer impedance, particularly the transfer impedance of structures, to assess vulnerability to lightning.
2. Background Art
Buildings of all types and their contents are vulnerable to the ever-present threat of lightning. High-value electronics, computers, communications systems, explosives, and human life are all particularly vulnerable to the effects of lightning. While many buildings have a metallic sub-structure that provides some protection against the effects of lightning, it is currently unknown how most buildings will fare in protecting their contents when struck. Hospitals, banks, bunkers for nuclear weapons, shelters for explosives, and buildings that house flight-control systems can all benefit from an analysis of their susceptibility to the effects of a lightning strike.
Past experimental and analytical work to assess the ability of typical explosives shelter to protect their contents from the effects of a direct lightning strike reveal several results. First, the typical shelter of reinforced concrete is intrinsically a very effective shield against a direct lightning strike if its mesh of rebar is electrically well bonded, that is, electrically well connected, rod-to-rod, mesh-to-mesh, and roof-to-floor. This means that the shelter itself conducts nearly all the lightning energy to ground with very little spectral energy transmitted into the shelter. If not well bonded, however, the shelter is an ineffective shield, and significant electric fields can be transmitted into the shelter. Second, the conventional lightning protection systems appliqued to shelters are ineffective and conduct only a small part of the lightning energy to ground. Third, the response of the shelter to lightning can be effectively determined from known lightning characteristics and the shelter transfer impedance from the roof to the floor. The important frequency range for this transfer impedance is about 1 kiloHertz (kHz) to 1 megaHertz (MHz). Indeed, this shelter impedance is well-modeled with a simple series R/L circuit having resistance (R) in the range of milliOhms and inductance (L) in the range of tens of nanoHenries. "Rocket-Triggered Lightning Studies for Protection of Critical Assets," M. E. Morris, et al., IEEE Transactions on Industry Applications, May/June 1994; "Sandia-Led Research May Zap Old Beliefs About Lightning Protection at Critical Facilities," J. German, Sandia Lab News, Apr. 25, 1997.
Patents in related technological fields include U.S. Pat. No. 4,328,461, to Butters, entitled "Apparatus for and Method of Measuring a High Voltage Electric Field," which discloses an apparatus for measuring a high voltage electric field, but does not disclose determination of transfer impedance. This technology does not disclose a dipole antenna, nor does it apply synchronous detection such as that of the present invention as will be discussed below. U.S. Pat. No. 4,142,143, to Daniel, entitled "Lightning Ground System Attachable Admittance Testing Instrument," discloses a lightning ground testing instrument but does not determine transfer impedance, nor does it have any system elements similar to the field probe of the present invention. U.S. Pat. No. 5,256,974, to Padden, entitled "Method and Apparatus for a Floating Reference Electric Field Sensor," discloses an apparatus for sensing electric fields with a dipole antenna, but does not determine transfer impedance or measure phase. U.S. Pat. No. 5,654,641, to Query, et al., entitled "Method and Device for Testing the Effectiveness of a Lightning Ground System," discloses measuring current or the magnetic field of an exposed conductor, but does not disclose the use of synchronous detection or any system element similar to the field probe of the present invention. U.S. Pat. No. 5,414,345, to Rogers, entitled "Apparatus and Method for Low Cost Electromagnetic Field Susceptibility Testing," discloses the use of a detector probe for monitoring a signal level at a test point as an AM radio frequency carrier. This system does not determine transfer impedance, nor does it contain any system elements similar to the unique field probe of the present invention. U.S. Pat. No. 5,414,366, entitled "Electromagnetic Field Susceptibility Test Apparatus and Methods," and U.S. Pat. No. 5,552,715, both also to Rogers, entitled "Apparatus for Low Cost Electromagnetic Field Susceptibility Testing," disclose similar technologies to that of the Rogers '345 Patent and also lack the same features from the present invention. All of these patents are dissimilar from the present invention in a variety of ways. For example, none disclose the ability to determine transfer impedance (both amplitude and phase), or the use of a fat half-dipole packaging for shielding and an electromagnetically invariant antenna configuration. None disclose the ability to use low drive signals and detect extremely low electric fields. None of the aforementioned devices are extremely immune to noise, or distribute optical isolation and battery power for explosives safety. Furthermore, the implementation of the present invention is optimized for impedance frequency ranges specific to lightning susceptibility.
The past experimental work that has been performed was with natural rocket-triggered lightning that produced direct strikes upon an instrumented shelter. However, the instrumentation and control equipment for this potentially hazardous experimental work required a cumbersome instrumentation trailer. FIGS. 3a, 3b and 3c represent a simplified view of a shelter response to lightning. An actual lightning strike produces a shelter drive current, I.sub.d, and an internal electric field voltage, V.sub.ef, as illustrated in FIG. 3a. For a poorly bonded shelter, V.sub.ef can be high enough to damage electronics or to actuate squibs or detonators. FIG. 3b illustrates the shelter transfer impedance, Z.sub.s, which analytically produces a closely similar internal field, V.sub.ef, if a similar drive current, I.sub.d, is applied to it. In FIG. 3b, V.sub.ef =Z.sub.s I.sub.d /h. FIG. 3c illustrates a low drive current measurement to determine Z.sub.s where Z.sub.s =V.sub.ef h/I.sub.d.